Don't use "line-through" style for completed items in remarkup checklists

Summary: Fixes T13482. Although this style makes physical sense by relationship to a written checklist, it seems to do more harm than good in practice.

Test Plan: Wrote a checklist with a checked-off item in remarkup, saw no more line-through.

Maniphest Tasks: T13482

Differential Revision: https://secure.phabricator.com/D20954